Biscuits for edge-gluing solid stock?
The only reason to use any type of reinforcement in joining solid wood into panels is to align the edges in order to minimize machining and unnecessary material loss.
I'd say that biscuits are a poor choice for alignment since they intentionally leave quite a bit of space/clearance between the biscuit and slot to allow the biscuit to swell when water-based glue is applied.
If your goal is actual joint strengthening, donít bother. The joints, if machined on properly dried lumber, using appropriate adhesive and clamping techniques, will not require any further reinforcement. The joint should be at least as secure as the surrounding wood if standard edge-gluing practices are followed.
Sounds like you may be finish-sanding your panels too soon after glue-up.
The wood swells more around the biscuit due to the larger amount of glue, and the swelling of the biscuit within the joint, as Michael mentioned. If you sand everything down flat before the area has a chance to completely dry, a depression is created at the biscuit when it does finally dry.
I agree with Michael, the biscuits probably won't add strength to a properly prepared edge joint. If you are just using them for alignment you might try not gluing the slots, and see if that helps.
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